Advice: don't try to mix it up with the Reds. You'll get your clock cleaned. It takes only a few turns of exciting, freewheeling action, Panzers vs Tank Armies, for the weight of the Red Army and its now monstrous Guards Rifle Corps to break the Panzer arm. Instead, you must mostly play for time. Withdraw some, most every turn, on most every sector. A withdrawal of a single hex tends to dislocate the attack of the infantry masses, and in winter, a withdrawal of just one hex can prevent some of those huge corps from launching any deliberate attack.
Any time you do decide to stand and make a fight of it (on a river line, say, or in a forest belt), keep Panzer reserves on hand and set to reserve, and keep every hex of your second and third line garrisoned. With an infantry regiment, if nothing else. What you cannot have is Russians just motoring into the rear and setting up pockets. Often, the first line isn't really the main line. It's a buffer zone. The Russians can attack it and drive you back, but you don't much care. The next hex is where you make your real stand.
River lines, of course, must be held where they are, and you might have to counterattack to restore them. But don't hang on too long. Remember what happened historically at Korsun.